The invention concerns equipment for playing back data that can be read out of the data tracks on a recorded medium by an optical pick-up in that at least one beam of light can be focused on the recorded medium by a focusing circuit and positioned on the data tracks by a tracking circuit, in that the beam of light is reflected onto a photodetector by the recorded medium, and in that the photodetector generates an electric data signal, a focusing-error signal as an actual value for the focusing circuit, and a tracking-error signal as an actual value for the tracking circuit.
Equipment of this type--compact-disk players, optico-magnetic equipment for recording and playing back, and recording and playback equipment for draw disks or videodisc players for example--are provided with an optical pick-up consisting of a laser diode, several lenses, a prismatic beam divider, and a photodetector. The design and function of an optical pick-up are described in Electronic Components and applications 6 (1984) 4, 209-15.
The beam of light emitted by the laser diode is focused by lenses onto the compact disk, which reflects it onto a photodetector. The data stored on the disk and the actual value for the focusing and tracking circuits are obtained from the photodetector output signal. The aforecited passage calls the actual value for the focusing circuit a focusing error and the actual value for the tracking circuit a radial tracking error.
The element that controls the focusing circuit is a coil with an objective lens that travels along its optical axis within the magnetic field. The focusing circuit moves the lens back and forth to keep the beam of light emitted by the laser diode constantly on the compact disk. The tracking circuit, which is often called the radial drive mechanism, moves the optical pick-up radially over the compact disk, positioning the beam of light on the spiral data tracks on the disk.
The radial drive mechanism in some equipment is comprises what is called a coarse-drive mechanism and of what is called a fine-drive mechanism. The coarse-drive mechanism is for example in the form of a spindle that radially moves the overall optical pick-up consisting of the laser diode, the lenses, the prismatic beam divider, and the photodetector. The fine-drive mechanism can tilt the beam of light radially, at a prescribed slight angle for example, allowing the beam of light, due to the tilting alone, to travel a short distance along one radius of the compact disk.
FIG. 1 illustrates the photodetector PD in the optical pick-up of a compact-disk player wherein three beams L1, L2, and L3 of light are focused on the compact disk. A pick-up of this type is called a three-beam pick-up in the aforecited reference.
The photodetector has four square photodiodes A, B, C, and D connected to create another square. Diagonally opposite this square comprising the four photodiodes are two other square photodiodes E and F.
The middle laser beam L1, which is focused on the four photodiodes A, B, C, and D, generates a data signal HF=AS+BS+CS+DS and a focusing-error signal FE=(AS+CS)--(BS+DS). The two outer beams L2 and L3 of light, of which forward beam L2 strikes photodiode E and rear beam L3 strikes photodiode F, generate tracking-error signal TE=ES--FS. Photodiodes A, B. C, D, E, and F generate photovoltages AS, BS, CS, DS, ES, and FS. Due to an astigmatic collimator lens in the path of middle laser beam L1 in the optical pick-up, the beam strikes the large square that comprises the four photodiodes A, B, C, and D in the form of a circle when it is precisely focused on the square and the form of an ellipse when it is out of focus.
FIG. 1a represents the device in focus and precisely tracked, which will be discussed again later herein. Because the spot of light produced on the large square by laser beam L1 is in the shape of a circle, focusing-error signal FE=(AS+CS)--(BS+DS)=0. The zero value of the signal informs the focusing circuit that the focus is precise.
FIG. 1b represents the device out of focus, with the compact disk too far from the objective lens. The focusing-error signal is negative: FE=(AS+CS)--(BS+DS)&lt;0. The negative value of the focusing-error signal informs the focusing circuit that the distance between the compact disk and the objective lens is too great. The adjustment mechanism in the focusing circuit accordingly moves the lens toward the disk until the focusing-error signal becomes zero.
FIG. 1c illustrates the opposite out-of-focus situation, with the objective lens too near the compact disk. The focusing-error signal has a positive value: FE=(AS+CS)--(BS+DS)&gt;0, informing the focusing circuit that the lens is too near the disk, and the adjusting mechanism moves the lens away from the disk until the focusing-error signal becomes zero.
How the tracking circuit carries out its function will now be described.
The laser beams L1, L2, and L3 illustrated in FIGS. 1a, 1b, and 1care precisely on track. The tracking-error signal has the value zero: TE=ES--FS=0.
FIG. 1d illustrates the situation in which laser beams L1, L2, and L2 are displaced to the right of the track. The tracking-error signal has a negative value: TE=ES--FS&lt;0. The adjusting mechanism in the tracking circuit moves the optical pick-up to the left until the tracking-error signal becomes zero.
In the opposite situation, when the laser beams are displaced too far to the left of the track, the tracking-error signal is positive: TE=ES--FS&gt;0, and the adjusting mechanism will move the optical pick-up to the right until the tracking-error signal becomes zero.
The relationship between the input parameter, the actual tracking-error signal, and the output voltage UA of the tracking-circuit control amplifier will be evident from the amplifier's characteristic curve in FIG. 2. The ideal operating point for the control amplifier is at the zero point, where the coordinates intersect, because that is where the operating point will be functioning symmetrically, traveling the same distance on either side. Prerequisite, however, to the desired ideal position of the operating point are photodiodes E and F that are exactly equivalent and ideal. Since, however, photodiodes E and F are not actually completely equivalent and are accordingly also subjected to different offset voltages, the operating point will, even though the lack of symmetry is corrected by the control amplifier, be displaced out of the ideal position and for example into the actual position AP represented in FIG. 2. The travel of the tracking-circuit control amplifier will now be limited to one side.